4,620 research outputs found
Quantum measurements of atoms using cavity QED
Generalized quantum measurements are an important extension of projective or
von Neumann measurements, in that they can be used to describe any measurement
that can be implemented on a quantum system. We describe how to realize two
non-standard quantum measurements using cavity quantum electrodynamics (QED).
The first measurement optimally and unabmiguously distinguishes between two
non-orthogonal quantum states. The second example is a measurement that
demonstrates superadditive quantum coding gain. The experimental tools used are
single-atom unitary operations effected by Ramsey pulses and two-atom
Tavis-Cummings interactions. We show how the superadditive quantum coding gain
is affected by errors in the field-ionisation detection of atoms, and that even
with rather high levels of experimental imperfections, a reasonable amount of
superadditivity can still be seen. To date, these types of measurement have
only been realized on photons. It would be of great interest to have
realizations using other physical systems. This is for fundamental reasons, but
also since quantum coding gain in general increases with code word length, and
a realization using atoms could be more easily scaled than existing
realizations using photons.Comment: 10 pages, 5 figure
Chaotic Crystallography: How the physics of information reveals structural order in materials
We review recent progress in applying information- and computation-theoretic
measures to describe material structure that transcends previous methods based
on exact geometric symmetries. We discuss the necessary theoretical background
for this new toolset and show how the new techniques detect and describe novel
material properties. We discuss how the approach relates to well known
crystallographic practice and examine how it provides novel interpretations of
familiar structures. Throughout, we concentrate on disordered materials that,
while important, have received less attention both theoretically and
experimentally than those with either periodic or aperiodic order.Comment: 9 pages, two figures, 1 table;
http://csc.ucdavis.edu/~cmg/compmech/pubs/ChemOpinion.ht
Quantum computation with mesoscopic superposition states
We present a strategy to engineer a simple cavity-QED two-bit universal
quantum gate using mesoscopic distinct quantum superposition states. The
dissipative effect on decoherence and amplitude damping of the quantum bits are
analyzed and the critical parameters are presented.Comment: 9 pages, 5 Postscript and 1 Encapsulated Postscript figures. To be
published in Phys. Rev.
Communication Steps for Parallel Query Processing
We consider the problem of computing a relational query on a large input
database of size , using a large number of servers. The computation is
performed in rounds, and each server can receive only
bits of data, where is a parameter that controls
replication. We examine how many global communication steps are needed to
compute . We establish both lower and upper bounds, in two settings. For a
single round of communication, we give lower bounds in the strongest possible
model, where arbitrary bits may be exchanged; we show that any algorithm
requires , where is the fractional vertex
cover of the hypergraph of . We also give an algorithm that matches the
lower bound for a specific class of databases. For multiple rounds of
communication, we present lower bounds in a model where routing decisions for a
tuple are tuple-based. We show that for the class of tree-like queries there
exists a tradeoff between the number of rounds and the space exponent
. The lower bounds for multiple rounds are the first of their
kind. Our results also imply that transitive closure cannot be computed in O(1)
rounds of communication
Heralded single photon absorption by a single atom
The emission and absorption of single photons by single atomic particles is a
fundamental limit of matter-light interaction, manifesting its quantum
mechanical nature. At the same time, as a controlled process it is a key
enabling tool for quantum technologies, such as quantum optical information
technology [1, 2] and quantum metrology [3, 4, 5, 6]. Controlling both emission
and absorption will allow implementing quantum networking scenarios [1, 7, 8,
9], where photonic communication of quantum information is interfaced with its
local processing in atoms. In studies of single-photon emission, recent
progress includes control of the shape, bandwidth, frequency, and polarization
of single-photon sources [10, 11, 12, 13, 14, 15, 16, 17], and the
demonstration of atom-photon entanglement [18, 19, 20]. Controlled absorption
of a single photon by a single atom is much less investigated; proposals exist
but only very preliminary steps have been taken experimentally such as
detecting the attenuation and phase shift of a weak laser beam by a single atom
[21, 22], and designing an optical system that covers a large fraction of the
full solid angle [23, 24, 25]. Here we report the interaction of single
heralded photons with a single trapped atom. We find strong correlations of the
detection of a heralding photon with a change in the quantum state of the atom
marking absorption of the quantum-correlated heralded photon. In coupling a
single absorber with a quantum light source, our experiment demonstrates
previously unexplored matter-light interaction, while opening up new avenues
towards photon-atom entanglement conversion in quantum technology.Comment: 10 pages, 4 figure
Independent individual addressing of multiple neutral atom qubits with a MEMS beam steering system
We demonstrate a scalable approach to addressing multiple atomic qubits for
use in quantum information processing. Individually trapped 87Rb atoms in a
linear array are selectively manipulated with a single laser guided by a MEMS
beam steering system. Single qubit oscillations are shown on multiple sites at
frequencies of ~3.5 MHz with negligible crosstalk to neighboring sites.
Switching times between the central atom and its closest neighbor were measured
to be 6-7 us while moving between the central atom and an atom two trap sites
away took 10-14 us.Comment: 9 pages, 3 figure
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